In recent years, high-density mounting of electrical circuits has been used widely as equipment is miniaturized and goes electronic. Accordingly, because of the increase in heat density of electronic equipment, a blower has been used for cooling the equipment. For a conventional blower, as shown in FIG. 15, an annular wall 2 is formed to be spaced from blade tips of an axial-flow fan 1, and the axial-flow fan 1 rotates around a shaft 4 in a blowing state in which a motor 3 is energized, by which an air flow 5 directed from the suction side to the discharge side is generated.
However, in the aforementioned blowing state, the velocity of the air flow increases on the suction side of the blade tips, and a low-energy zone due to the effect of an inter-blade secondary flow is produced on the blade trailing edge side where the air flow is converted into pressure energy. In this zone, a loss is great, and the flow is liable to be separated. The air flow is separated from the blade surface, so that vortexes are produced in the separation region, by which turbulence noise is increased, and the noise level and the static pressure vs. air quantity characteristics (hereinafter referred to as P-Q characteristics) are deteriorated. This phenomenon is frequently found especially when a flow resistance (system impedance) is applied to the discharge flow side and when the occurrence of leakage vortexes at the blade tips increases, by which the fan gets into a stalling state. As a blower in which the shape of the annular wall provided at the outer periphery of the fan is devised to improve such fan characteristics, blowers described in Japanese Patent Application No. 8-174042, Japanese Patent Application No. 9-151450 and Japanese Patent Application No. 9-260738 have been proposed by the same applicant as the one of the present invention. Also, National Publication of International Patent Application No. 6-508319 and U.S. Pat. No. 5,292,088 have disclosed blowers in which a plurality of ring bodies are arranged at intervals at the outer periphery of an axial-flow fan, so that vortexes of air flowing in through gaps between the ring bodies increase the flow rate of fluid. Further, U.S. Pat. No. 5,407,324 has disclosed a blower in which a plurality of annular plates surrounding the outer periphery of an axial-flow fan are stacked and the inner peripheral portion of said annular plates being inclined along the direction of air, whereby the flow of air between the inner periphery and the outer periphery of the annular wall is enabled. In all of these blowers, the fan characteristics are improved by the suction of air from the outer periphery of the fan.
However, for a rectangular blower having an external shape ranging from about 60 mm.times.60 mm to about 92 mm.times.92 mm, which is used for personal computers, workstations and the like, the shape, dimensions, etc. are made common to reduce the cost, so that a large change so as to make the external shape circular is undesirable. To improve the characteristics of a blower having an outer peripheral shape other than circular one, Japanese Patent Application No. 9-151450 and Japanese Patent Application No. 9-260738 filed by the same applicant as the one of the present invention have disclosed a method for improving the characteristics by providing slits in the annular wall and changing the width of a slit gap. FIGS. 16 to 18 show a blower disclosed in Japanese Patent Application No. 9-151450. As shown in FIG. 16(b), the total width of stacked annular plates 7a to 7d is set so as to be the same or almost the same as the width in the axial direction of an axial-flow fan 1. Also, the width w of a gap of each slit 6 is changed continuously so that the inflow resistance at each portion is equal. FIG. 18 schematically shows a case where the width w of the gap of the slit 6 is constant over the entire circumference. When the axial-flow fan 1 is rotated in the direction of an arrow 9, a negative pressure is produced on the suction side at the blade tips, so that an air flow 5 toward the inside through the slits 6 is generated by a difference in pressure between the inside and the outside. By setting the width w of the gap of the slit 6 at an appropriate value, the air flow 5 flowing in through the slits 6 is made a laminar flow, so that leakage vortexes 10 flowing from the pressure side to the suction side at the blade tips are restrained, by which the separation of the air flow on the suction side surface is eliminated. In this case, however, the slits at four-side portions 7s have a lower air inflow resistance than the slits at other portions 7r, so that the air inflow quantity at the four-side portions 7s becomes larger than that at the other portions 7r. Therefore, the air flow at this portion is prone to become a turbulent flow, and at the same time, a portion having a high flow rate and a portion having a low flow rate are produced on the fan, causing the blade to vibrate, or a disk circulation 12 is easily generated such that the air flows backward from the downstream-side slit and is sucked again into the upstream-side slit, which deteriorates the P-Q characteristics and causes noise to increase. By contrast, FIG. 17 shows a case where the width w of the gap of the slit 6 is changed continuously so that the inflow resistance at each portion is equal. In this case, both of the slits at four-side portions 7s and the slits at other portions 7r have an equal air inflow resistance, so that the air inflow quantity is equal over the entire circumference, which restrains blade vibrations, disk circulation, etc., and eliminates the deterioration of the P-Q characteristics and the increase of noise.
However, in the aforementioned technology, the width w of the gap of the slit 6 is assumed to be constant in the radial direction, so that the radial cross section of the annular plate 7a to 7d is inevitably rectangular. By this configuration, although the P-Q characteristics are greatly improved by the above-described effect, regarding the noise, the annular wall itself, which is provided with slits, becomes a new noise source. Under such a service condition that a great stall does not occur even in a conventional blower, particularly at a low pressure, the noise sometimes increases on the contrary.
An object of the present invention is to further improve the shape of a slit portion and especially to reduce the noise in a blower in which an annular wall as described above is formed with slits which provide communication between the inner peripheral portion and the outer peripheral portion, and air is sucked into the inner peripheral portion of the annular wall through the slits as a fan rotates.